In recent years, in the automobile industry, metal pipe is increasingly being used as one means for reducing weight. Hollow metal pipe, compared with a solid material, offers the same rigidity while enabling the cross-sectional area to be reduced. Further, an integral structure of metal pipe, compared with a T-shaped structure obtained by welding two metal plates, enables a reduction of weight by the elimination of the need for a welded flange part.
However, auto parts are placed in narrow spaces in the automobiles. Therefore, metal pipe is seldom used as is as a straight pipe. It is almost always attached after being secondarily worked. As secondary working, bending is used most often, but in recent years the increasing complexity of the shapes of auto parts has led to an increase in hydroforming as well (fastening a metal pipe in a mold and, in that state, using inside pressure and axial direction compression to work the pipe into the mold shape) and, further, an increase in working comprised of these working processes overlaid. Hydroforming itself, as shown in FIG. 1 (see Journal of Materials Processing Technology, Vol. 45, No. 524 [2004], p. 715), compared with the simple T-forming, is being used for increasingly complex shapes in recent years. The pipe expansion rates (ratio of circumferential length of product pipe to circumferential length of stock pipe, in the figure, described as “expansion ratio”) have also been increasing.
As the method of hydroforming with a large expansion ratio, as for example described in Japanese Patent Publication (A) No. 2002-153917, there is the method of using a movable mold to obtain a hydroformed part having a high branch pipe height. However, this method can only be applied to shapes in the case of expansion in only a certain direction such as with T-forming.
Further, Japanese Patent Publication (A) No. 2002-100318 discloses the method of expansion in one certain direction, then expansion in a direction perpendicular to that direction. If using this method, it is possible to obtain a hydroformed part expanded not only in one certain direction, but overall. However, while this can be easily applied if expanding the pipe to a simple rectangular cross-section, if a complicated cross-sectional shape, a further hydroforming step becomes necessary for finishing the part to the detailed shape. A total of three steps of hydroforming become necessary.
If performing both bending and hydroforming, in general the part is bent, then loaded into the hydroforming mold and hydroformed, but with this method, it is difficult to increase the expansion ratio of the bent part. Therefore, the method of hydroforming, then bending is also proposed in for example Japanese Patent Publication (A) No. 2002-219525. This method expands the pipe overall in the first step of hydroforming, then bends it while applying internal pressure in the second step, and finally hydroforms the part while crushing it in the direction perpendicular to the bending direction in the third step. If using this method, compared with the general method of bending, then hydroforming, it becomes possible to increase the expansion ratio of the bent part. However, the expansion ratio is limited by the limit value of the first step of hydroforming. With hydroforming expanding the pipe overall like with this method, not that large an expansion ratio can be expected.
In addition, as in Japanese Patent Application No. 2006-006693, the method of hydroforming, then rotary bending has also been proposed. However, with this method, the scope of application is limited since only rotary draw bending is covered as a bending method.